1. Field of the Invention
The present invention relates generally to a fiber optic sensing coil. More particularly, the present invention relates to a fiber optic sensing coil that reduces measurement error, increases LD product (a product of the length and diameter of the coil) for the same volume, improves LD stability, and simplifies manufacturing processes.
2. Background of the Invention
A fiber optic sensing coil is commonly used in fiber optic rotation sensing devices, such as an interferrometric fiber optic gyroscope (IFOG). As is well-known, the fiber optic sensing coil is a continuous optical fiber wound in a circular or looped shape that acts as a sensing device to detect a Sagnac phase difference for two counter-propagating beams in presence of rotation.
FIG. 1 shows a conventional optical fiber. As shown, optical fiber 10 typically consists of three main elements. A core 12 is a central section of the optical fiber and is the light transmitting region of the fiber and is usually made of silica. A cladding 14 is an optical region that surrounds core 12 and is usually made of silica or hard clad silica materials. Finally, a coating 16 that has non-optical properties surrounds cladding 16. Coating 16 protects the silica structure (i.e., the core and the cladding) from physical and environmental damages. Typically, coating 16 includes one or more layers of a polymer composition.
FIG. 2 shows a cross sectional view of a portion of a conventional fiber optic sensing coil. The sensing coil includes a continuous optical fiber 22 wound upon a supportive spool 24. The windings of optical fiber 22 are potted within a matrix of adhesive bonding material 26.
FIG. 3 shows an enlarged view of a portion of the wound coil taken at region 3 of FIG. 2. As shown, optical fiber 22 includes a core 30, a cladding 32 around core 30 and a coating 34 around cladding 32. As mentioned earlier, typically, core 30 is made of silica, cladding 32 is made of silica or hard clad silica material, and coating 34 is made of one or more layers of a polymer composition.
Many factors affect performance of a sensing coil, thereby affecting performance of a sensing device. For example, the product of the length and diameter (LD product) of the wound fiber optic sensing coil affects the sensitivity of a sensing device. Generally speaking, the greater the LD product, the greater the sensitivity and stability of the sensing device, and the lower the random noise. However, large coil volume resulting from greater fiber length or larger loop diameter (to achieve a higher LD product) requires increasingly bulky packages for the sensing device.
Also, as is well known, when the core material has different physical properties as compared to the coating material of the optical fiber, anisotropic thermal stresses can be induced in the sensing coil, thereby introducing a bias error. Further, environmental factors, including variables such as temperature, vibration (acoustical and mechanical) and magnetic fields, can affect the measured phase shift difference between the counter-propagating beams, thereby introducing a further bias error.
In attempts to reduce the bias errors introduced by such factors, and thereby enhance the accuracy of the sensing devices, various symmetric coil winding configurations have been proposed. For example, U.S. Pat. No. 4,793,708 (Fiber Optic Sensing Coil), which is incorporated herein by reference, teaches a symmetric fiber optic sensing coil formed by dualpole or quadrupole winding, and U.S. Pat. No. 6,215,933 (Bifilar FOG Coil Winding Pattern with Improved Shupe Bias Canceling Properties), which is incorporated herein by reference, teaches a bifilar coil winding method. The bifilar coil winding method employs two identical parallel fibers, each one-half the length of the final coil wound between first and second spools so that the respective midpoint of each fiber is located between the two spools. Both parallel fibers are then wound to form a coil using the quadrupole or dipole winding technique.
However, while the advantages of symmetric windings are recognized, this desirable arrangement has proven difficult to realize and there continues to be a need to improve the performance of sensing coils.